Treatment of plaster of paris bonded molds with liquid hydrocarbon prior to autoclaving



Patented Sept. 6, 1949 TREATMENT OF PM El) MOLDS WITH STER OF PARISBOND- LIQUID HYDROCARBON PRIOR TO AUTOCLAVING Xarii'a L. Bean,

Serial No.

'1 Claims. (Cl. 22-192) This invention relates generally to thetreatment of plaster molds for use in metal casting and moreparticularly to the treatment of plaster of Paris bonded molds processedin accordance with the process of Bean Patent 2,220,703, dated November5, 1940.

According to the process of the Bean patent, after the initial set ofthe wet plaster mix comprising essentially water, plaster of Paris, andfinely divided re fractry material, the green plaster mold is placed inan autoclave and subjected to steam under pressure to convert calciumsulfate di-hydrate to calcium sulfate hemi-hydrate and to leave freewater distributed through the plaster. The mold is thereafter removedfrom the autoclave and allowed to stand for a number of hours at roomtemperature whereby the hemihydrate is rehydrated to (ii-hydrate. Themold is then dried to remove the free water from the plaster. Plasterbonded molds processed according to Patent 2,220,703 become granular instructure on the interior, while the surface, in general, remains finegrained and smooth. The effect of the treatment is to increase thepermeability of the material so as to permit escape of gases generatedin the mold during casting while providing a smooth skin at the surfaceof the mold giving better definition to the surface of the casting.

At times some difliculty has been experienced with plaster bonded moldsin accordance with the Bean patent in that the surface of the moldsbecome granular, especially in grooves, and places where the amount ofevaporation from the surface is less than ordinary. The primary objectof this invention is to provide a relatively simple and inexpensivemethod of treating plaster of Paris bonded molds so as to eliminate thetendency for forming the granular material at the mold surface.

I have discovered that treating the green mold surface after the initialset of the wet plaster mix and before autoclaving, by applying a liquidhydrocarbon thereto protects the mold surface during the subsequentsteps and inhibits formation of granular material on the surface. Themold treated with the liquid hydrocarbon is then given the regularprocessing in accordance with the Bean patent.

The liquid hydrocarbon may be applied to the green mold surface in anydesired manner so long as the mold surface is thoroughly wetted thereby.Preferred practice is simply to dip the green plaster mold in the liquidhydrocarbon for a short time. The time of immersion is not particularlycritical. In general. prolonged immer Yellow Springs, Ohio, assignor toGeneral Motors Corporation, corporation of Delaware No Drawing.Application Detroit, Mich a September 18, 1947,

sion has no particular adverse efiect, though immersion of much over 5minutes probably should be avoided because of possible damage to thesurface of the mold and because the longer times of treatment areunnecessary.

An example of a preferred liquid hydrocarbon is one consisting of 10% ofparaflin oil and kerosene. The percentage of parafiln oil is notespecially critical. The foregoing combination is recommended because ofits cheapness and general availability and because of the fact thatkerosene alone is not completely effective in all cases where thetreated green molds stand around for several hours before they are giventhe autoclave treatment. This is probably due to the kerosene tending towick into the body of the mold andaway from the mold surface. Theparafiin oil is a light oil designated as fmold oil by all the major oilcompanies which sell the same to foundries, etc., as a mold releaseagent. The mold oil is a light parafiin oil and is understood to containa small percentage of lard oil.

Kerosene alone has been used but, as noted above, is not as satisfactoryas the combination which includes the small proportion of paraflin oil.Diesel distillate which is quite similar to kerosene may be used eitherby itself or with a small proportion of paraffin'oil as in the examplesgiven in which kerosene is used. Other liquid hydrocarbons havingproperties generally similar to the examples given above may beemployed. However, as indicated above, the materials of the specificexamples are preferred because of commercial availability and cheapness.

The dipping of the green mold is usually done within a time interval notgreater than six hours before the mold is placed in the autoclave.However, the use of the combination including kerosene and paraffin oilallows even more latitude in elapsed time-for example, up to twenty-fourhours.

After dipping the molds are set on a rack. What kerosene does not drainofi in the first few seconds is quickly absorbed into the mold.

The green plaster of Paris bonded mold after being treated with theliquid hydrocarbon is subsequently given the autoclave treatment withsteam un der pressure to convert the calcium sulfate di-hydrate tocalcium sulfate hemi-hydrate and to leave free water distributedthroughout the mold. The mold is then removed from the autoclave andallowed to stand for a number of hours whereby the hemi-hydrate isrehydrated to dihydrate. The mold is then dried to remove all orsubstantially all of the remaining free water from the plaster. Theautoclaving and subsequent steps are in accordance with the teachings ofthe Bean patent.

The treatment with the liquid hydrocarbon in accordance with the presentinvention has the important effect that it inhibits granule formation atthe very surface without adversely affecting the recrystallization ofthe interior. Many inhibitors prevent the surface from becoming granularbut also prevent recrystallization of the interior wherever theypenetrate the surface, thus destroying most of the value which otherwisewould be obtained by the autoclaving and other treatment of the Beanpatent.

' While the specific embodiments of the invention described herein dealwith the formation of plaster of Paris bonded molds, it will beunderstood that the principles of the invention have application also inthe formation of plaster of Paris bonded cores processed in accordancewith Bean Patent 2,220,703.

Various changes and modifications of the embodiments of my inventiondescribed herein may be made without departing from the principle andspirit of the invention.

I claim:

1. The process of forming a plaster of Paris bonded mold or core whichincludes forming a desired shape from a mixture comprising essentiallywater, plaster of Paris and finely divided refractory material, settingthe plaster, immersing the set plaster shape in a liquid hydrocarbonconsisting of about 90% kerosene and of light paraffin oil, removingfrom said liquid hydrocarbon the plaster shape, thereafter dehydratingthe set plaster shape as thus treated to convert calcium sulfatedi-hydrate to calcium sulfate hemi-hydrate and to leave free waterdistributed throughout the shape, thereafter rehydrating thehemi-hydrate to di-hydrate and then drying the shape to remove the freewater from the shape.

2. In a process of forming a plaster of Paris bonded mold or core byforming a desired shape from a mixture comprising essentially plaster ofParis, finely divided refractory material and water, setting theplaster, dehydrating the set plaster to convert calcium sulfateiii-hydrate to calcium sulfate hemi-hydrate and to leave free waterdistributed throughout the shape, thereafter rehydrating thehemi-hydrate to di-hydrate and then drying the shape to removesubstantially all of the free water therefrom, the improvement whichconsists in thoroughly wetting the surface of said set plaster shapeprior to the dehydrating step with a light liquid hydrocarbon.

3. A process as in claim 2 in which the liquid hydrocarbon consistsessentially of kerosene.

4. A process as in claim 2 in which the liquid hydrocarbon oil consistsessentially of Diesel distillate.

5. A process as in claim 2 in which the liquid hydrocarbon consistsessentially of kerosene and contains a small percentage of lightparaffin oil.

6. In a process of forming a plaster of Paris bonded mold or core byforming a desired shape from a mixture consisting essentially of plasterof Paris, finely divided refractory material and water, setting theplaster, dehydrating the set plaster to convert calcium sulfatedi-hydrate to calcium sulfate hemi-hydrate, thereafter rehydrating thehemi-hydrate to di-hydrate and then drying the shape to removesubstantially all of the free water therefrom, the improvement whichconsists in thoroughly wetting the surface of said set plaster shapewith liquid hydrocarbons having properties substantially similar tothose of kerosene prior to said dehydrating step.

7. The process of forming a plaster of Paris bonded mold or core whichcomprises forming a desired shape from a mixture consisting essentiallyof plaster of Paris, finely divided refractory material and water,settirg the plaster, immersing the set plaster shape in a hydrocarbonliquid consisting essentially of kerosene, removing from saidhydrocarbon liquid the plaster shape, thereafter dehydrating the setplaster shape as thus treated to convert calcium sulfate di-hydrate tocalcium sulfate hemi-hydrate and to leave free water distributedthroughout the plaster shape. thereafter rehydrating the hemi-hydrate todihydrate and then drying the shape to remove the free water from theshape.

XARIFA L. BEAN.

REFERENCES CITED UNITED STATES PATENTS Name Date Bean Nov. 5, 1940 OTHERREFERENCES Nature, November 14, 1931, vol. 128, page 838, article byMaurice Copisarow.

Number

